Abstract

Ecological communities are structured by multiple biotic and abiotic forces. Temporal variations in external forces result in communities that vary temporally at multiple scales. Aim of this study was to examine temporal variations in phytoplankton communities in two small adjacent eutrophic lakes with contrasting disturbance regimes; one is shallow and constantly mixing and the other one is stratified and thus more stable. Analyses of classification strength showed that in the more stable lake, community composition differed more among the years than in the more disturbed lake. However, according to Multi-Response Procedures, communities were more variable within years in the more disturbed lake. According to Canonical Correspondence Analysis, community composition was most strongly controlled by variation in nitrogen and iron concentrations, pH, Secchi-depth and nitrogen to phosphorus ratio. We also found a strong negative relationship between diversity and biomass production in both lakes. Our results support the view that under very disturbed conditions, the assembly follows more the same trajectory among the years, as only a limited number of species persists and the number of species occupying the lake throughout the years may be lower. This gives evidence that physical forcing is a highly important driver for the phytoplankton assembly resulting in clear differences in communities between the adjacent lakes.